An exposure apparatus for semiconductor manufacturing is one that projects and transfers a circuit pattern formed on the surface of an object, such as a photomask (hereinafter, simply "mask"), onto a substrate, such as a wafer, through an image-forming apparatus, such as a projection lens. The substrate is coated with a light-sensitive material, such as photoresist. Upon exposure of the mask, a photoresist pattern is obtained on the substrate. To obtain a photoresist pattern over a desired area (i.e., exposure field), the mask must be illuminated by light having a uniform intensity and a uniform divergence angle. Accordingly, the illumination apparatus of such exposure apparatus have employed Kohler illumination to satisfy these conditions.
If the exposure light is X-rays, then the image-forming apparatus comprises a reflector. An off-axis circular arc-shaped (i.e., arcuate) exposure field is used, so that only an arcuate area on the mask is projected and transferred onto the wafer in a static exposure. Accordingly, the transfer of the circuit pattern on the entire mask onto the wafer is performed by simultaneously scanning the mask and wafer in fixed directions.
In a scanning-type exposure, it is desirable that the illumination optical system uniformly illuminate the entire arcuate area on the mask at a fixed numerical aperture. An illumination optical system that can accomplish this is disclosed in Japanese Patent Application Kokai No. Hei 7-235471, applied for by the present applicant.
The optical system disclosed in the above-mentioned Japanese Patent Application is shown herein in FIG. 6 and FIG. 7. X-rays 120, comprising beams 121 and 122, are emitted from light source (or light source image) 110 and are reflected by a special reflector 130, thereby forming convergent beams 124 and 125, respectively, which irradiate an arcuate area 140 on the mask (not shown). Arcuate area 140 is centered about a point 144 (see FIG. 6), and an X-Y-Z coordinate system is shown for reference.
As an example of a method of forming a light source for an illumination optical apparatus, an illumination apparatus of high illumination intensity is disclosed in Japanese Patent Application Kokai No. Hei 8-148414, applied for by the present applicant. With reference now to FIG. 8, illumination optical system 100 comprises an excitation energy light generation unit 101, a target member 103, and an illumination optical system 104 as the principle components. Excitation energy light rays 102 emitted from unit 101 irradiate a plurality of locations 110 on target member 103. X-rays 120 are respectively generated from locations 110, thereby forming a plurality of X-ray sources 110 (i.e., locations 110 become X-ray sources 110).
With reference now to FIGS. 9, 10a and 10b, parallel x-ray beams 121 and 122 are emitted from sources 110 in the sagittal direction (i.e., in the plane of the paper). When the emission angle .theta. is 0 degrees, beam 121 has a diameter p(.theta.)=q. When the emission angle is .theta., beam 122 has a diameter p(.theta.)=q.multidot.cos .theta.. Light beam diameter p(.theta.) (in the plane of the paper) decreases as emission angle .theta. increases. Accordingly, with reference now to FIGS. 10a and 10b, the cross section of beam 121 when the emission angle is 0 degrees is nearly circular (see FIG. 10a). This is in contrast to the cross section of beam 122, which cross section is elliptical when the emission angle is .theta. (see FIG. 10b). Beam 122 cross section (FIG. 10b) has a major axis p(0) in the meridional direction (i.e., perpendicular to the plane of the paper in FIG. 9) and a minor axis p(.theta.) in the sagittal direction.
With reference now to FIG. 11, when parallel beam 121, having an emission angle of 0 degrees (see FIG. 9), is subject to the converging action of reflector 130 (see FIG. 7), convergent beam 124 is formed. Beam 124 is conical and constantly extends an equal angle with respect to convergence point P1 in an arcuate illumination area (field) BF formed on the object (not shown) to be irradiated. In contrast, when parallel light beam 122, having an emission angle of .theta. (see FIG. 9) is subject to the converging action of reflector 130 (see FIG. 7), convergent light beam 125 is formed. Beam 125 converges in an elliptical spindle-shape at convergence point P2 in arcuate illumination area (field) BF on the object (not shown) to be irradiated.
Consequently, in the radial direction R at convergence point P2, the angle that convergent beam 125 extends with respect to convergence point P2 is equal to that of parallel beam 121 mentioned above. However, in the tangential direction T at convergence point P2, the angle that convergent light beam 125 extends with respect to convergence point P2 is smaller than that in the radial direction R at convergence point P2 (a multiple of cos .theta.). In addition, this effect is pronounced for parallel light beams with a large emission angle .theta. with respect to the sagittal direction. Thus, if an object is illuminated by an illumination apparatus which forms convergent light beams of a different cross-sectional shape, and an image of the object is formed by an image-forming apparatus, then the resolution of the image thus formed is generally not uniform over the image plane (exposure field). This is because a portion of the object is illuminated under conditions that do not satisfy the numerical aperture required by the image-forming apparatus.
Japanese Patent Application Kokai No. Hei 6-267894 discloses a method to solve the above-described problem by using a new image-forming optical system. However, since this optical system comprises a plurality of lenses, it is not useful in the X-ray region wherein lenses cannot be used. In addition, even the optical system disclosed therein comprised reflectors, the amount of X-rays obtained after reflection would be extremely small, since a plurality of reflectors would be necessary.